1. Field of the Invention
This invention relates to the field of integrated circuit packaging, and especially to a method and device for manufacturing tape packaged integrated circuits.
2. Description of the Prior Art
Tape automated bonding (TAB) is a well known method of attaching integrated circuit die to their electrical leads. TAB involves attaching (typically with epoxy adhesive) the individual die to a strip of Kapton flexible tape having copper traces (usually gold plated or solder plated) formed on the tape. The bond pads on the die are bonded to the copper traces, either directly (referred to as B-TAB) or by using gold bumps on the bond pad (TAB). The die is then encapsulated (i.e., packaged), then excised (i.e., cut) from the tape, with the traces on the tape serving as the leads for later electrical interconnection by the user of the integrated circuit. Thus there are no rigid metal leads provided at all, unlike the typical DIP package.
The tape is typically 5 mil (0.13 mm.) thick Kapton with 1.5 mil (0.004 mm.) thick copper traces on it. The tape is very flexible, with no structural strength. The tape is typically provided on a reel, and takes a "set", (i.e., a curl) from being on the reel, making it difficult to flatten the tape to properly bond the die. Thus it is easy to crack the die or have the die "pop off" due to the curl.
For simple integrated circuits having low lead counts, such as twenty or fewer leads, the tape is handled in reel-to-reel fashion with the die attached during later processing. Such an approach is not feasible for larger more complex die with more leads, since the tape cannot be held flat and straight enough using known methods.
Thus, for high lead count die, the prior art typically cuts the Kapton tape into short strips, each strip long enough to accommodate only three or four die.
An alternative prior art method involves cutting the tape into very short strips, each long enough for only one die, and putting the short strip of tape in a single slide carrier, which is a small plastic frame for further handling (i.e., bonding the tape to the die). A major problem with this approach is that the plastic frame is not conductive, so the leads on the tape which are already bonded to the die are not shorted together, causing an ESD (electrostatic discharge) problem which means that static electricity can easily damage the die.
The slide carriers have a further disadvantage that the leads must be plated in a solder electroplating bath so as to be solderable by che user, but the plastic slide carriers are softened and thus damaged by exposure to the electroplating bath.
Another deficiency of the prior art is that, as described above, the tape is supplied with its traces usually either solder plated or gold plated. Gold plated leads are typically used for wire bonding, or for thermal compression bonding. Solder (i.e., tin and lead) plated leads are typically used for a gold-eutectic bond to the die. If there is any gold on the solder-plated type tape, gold embrittlement undesirably occurs in the solder, and the thermal cycling in later processing causes bonding failures due to the embrittlement.
In conventional TAB bonding, as used for low lead count packages with relatively simple IC's, the inner ends of the copper traces on the tape are bonded directly to gold plated "bumps" on the die at each die bond pad. The copper traces are bonded to the bumps by thermocompression or gold-tin (i.e., solder) eutectic methods. The gold plating on the copper traces enhances the bond to the gold plated bumps on the die.
Another known bonding method for VLSI purposes (i.e., high lead count packages) uses the same kind of tape as described above, but attaches the die connect pads not directly to the copper traces, but instead by means of wire bonds. In this method very short wires of aluminum or gold are attached at one end to the connect pads on the die and at their other end to the inner ends of the copper traces on the tape. The wires are attached usually by thermocompression bonding, ultrasonic bonding, or thermosonic bonding.
Also well known in the art are lead frames. Lead frames are sacrificial metal members which hold metal leads in position during the encapsulation phase of semiconductor packaging. The lead frames are cut away and discarded after packaging is completed, but serve to enhance handling during packaging. Lead frames are not conventionally used in conjunction with tape packaging, since the tape itself serves the purpose of holding the traces in position.